Somatic hybrids of ASL-1 murine leukemia cells and LM(TK) cells, a sustained mouse cell line, express tumor antigens immunologically crossreactive with a similar tumor associated antigen formed by ASL-L cells. Although the hybrids are neoplastic in immunologically deficient animals, they are rejected after forming a localized tumor in immunocompetent syngeneic (A/J x C3H/HeJ)F1 mouse recipients. Mice rejecting hybrid cell tumors are partially and in some instances completely resistant to otherwise lethal injections of the leukemia cell line used as one parent in forming the hybrid. One hundred percent of animals not injected previously with ASL-1 x LM(TK)- hybrid cells die of leukemia if injected with ASL-1 cells. I propose to investigate the use of syngeneic hybrid cells in the treatment of spontaneously occurring and passively transferred murine leukemia. Somatic hybrids of ASL-1 and LM(TK)- cells are prepared with the aid of polyethylene glycol. Fused cells selected in medium containing hypoxanthine, aminopterin and thymidine (HAT) may be cultivated indefinitely in vitro. They share the histocompatibility antigens of both parents and a hybrid karyotype. Cellular immune mechanisms are involved in the resistance of mice rejecting hybrid cells to the successful transfer of ASL-1 cells. Cells from the spleens of mice rejecting hybrid cells inactivate ASL-1 leukemia cells in vitro and stimulate the release of 51Cr from prelabeled ASL-1 cells. The reaction is specific for the leukemia cell line used as one of the parents in forming the hybrid. Syngeneic mice receiving ASL-1 cells simultaneously with or followed by ASL-1 x LM(TK)- hybrid cells survive for significantly longer periods than mice receiving ASL-1 cells alone. This finding is of especial interest because it occurs entirely in an inbred mouse system in which histocompatibility differences between leukemic and hybrid cells are likely to be confined to immunologically crossreactive tumor-associated antigens expressed by neoplastic leukemia cells and by non-neoplastic viable hybrid cells.